This invention relates to atmospheric absorption spectroscopy, and more particularly to digital control of a tunable diode laser (TDL) in the atmosphere from a ground station.
Spectrometers utilizing diode lasers have been made available for analytical use in laboratories. Typically, they provide spectral resolution over a limited range in the 330 to 3600 cm.sup.-1 infrared spectral region. These spectrometers are useful in a variety of high resolution experiments, which include low level trace species detection.
The measurement of trace species can also be accomplished in the atmosphere using a TDL instrument, provided the diode laser can be remotely tuned through the desired wavelength region for interaction with specific atmospheric species. The potential of an instrument containing several TDLs, each of which can be separately tuned to simultaneously measure several reactive gaseous species in the stratosphere, makes it particularly attractive for stratospheric photochemistry. The application demands stable output wavelengths and the ability to tune the lasers with high precision, due to the fact that, at the low stratospheric pressures, the absorption lines are very narrow.
An instrument which utilizes high resolution spectroscopy with TDLs must have such accurate frequency control and high stability requirements as to present a challenge to the designer, even in a fairly well controlled laboratory environment. (This is because the output wavelength of a TDL varies as a function of temperature over a significant region). With a required temperature set-point resolution on the order of milli-Kelvins (10.sup.-3 K.), and stability requirements of a fraction of a milli-Kelvin, factors usually ignored become quite noticeable.
In a commercial TDL temperature controller designed for laboratory use, a refrigeration unit utilizes pressurized helium gas in a closed-cycle cooler, referred to as the cold head, to maintain the cryogenic temperatures of the TDL's environment. Silicon diode sensors sense the temperature of the TDL heat sink, referred to as the cold finger, to provide a feedback signal for precise temperature control of the TDLs using an electrical heating element to stabilize the cold finger temperature.
The TDLs are tuned within a particular wavelength region by controlling the operating temperature at a set point, typically between 15.degree. K. and 100.degree. K., and by controlled changes in the currents passed through the TDLs. Temperature tuning rates are typically 4 cm.sup.-1 /.degree.K. Current tuning rates are typically between 2-5.times.10.sup.-3 cm.sup.-1 per milliamp. In practice, a stable TDL current control voltage source is divided down to the set-point level through a series of high quality potentiometers. The potentiometers are manually adjusted to the desired setting by the user. A TDL temperature controller having its own voltage source divided down to a temperature set through a series of high quality potentiometers then stabilizes the temperature. A commercial instrument based on these principles has been used extensively in laboratories with good results, but it is not suitable for certain environments where extreme stability and remote control are required.
The instrument for atmospheric studies must be self-contained, and linked to the user via an RF link. Therefore all settings, adjustments, and data must be communicated through this RF link. Since the ambient temperature and pressure change considerably during flight, thermal stability of the control and monitoring system is very important.